Nociception, or the sense of noxious stimuli, is essential for our daily lives. Normal acute nociception prevents us from potential damage or repetitive injuries, while distorted neural circuits in pathological conditions gener- ate chronic pain, which is a huge human health problem. At present, our understanding of neural circuits in mediating and modulating pain sensation under normal and pathological conditions is surprisingly incomplete. We proposed to study a population of inhibitory dorsal spinal cord interneurons, which express the receptor tyrosine kinase RET neonatally and makes up about one third of inhibitory interneurons in laminae III to V (deep layer). Our preliminary study showed that these deep layer early RET+ inhibitory interneurons are unique and their circuits and functions in nociception have not been defined before. Aim 1. Define molecular, physiological, and anatomical properties of deep layer early RET+ inhibitory interneurons. In this aim, we will genetically label deep layer early RET+ inhibitory interneurons to study their gross anatomy, identities of inhibitory neural transmitter, physiological properties, and single neuron morpholo- gy. Our anticipated results will reveal unique features of deep layer early RET+ inhibitory interneurons and pro- vide an insight into their potential connections and functions. Aim 2. Dissect neural circuits associated with deep layer early RET+ inhibitory interneurons. In this aim, we will use both light/electronic microscopy imaging and spinal cord slice recording coupled with electric and optical stimuli to determine input and output of deep layer early RET+ inhibitory interneurons. Together, our work will reveal functional connections associated with this new population of DH inhibitory interneurons. Aim 3. Determine functions of deep layer early RET+ inhibitory interneurons in acute pain and chronic pain. In this aim, we will either ablate deep layer early RET+ inhibitory interneurons using toxin or acutely acti- vate them using optogenetic and pharmacological approach and test mouse nociceptive behavioral responses under acute and chronic pain conditions. With these experiments, we anticipate revealing important functions of deep layer early RET+ inhibitory interneurons in modulating acute and chronic pain. In short, our proposed study will elucidate circuits and function of a new population of DH inhibitory interneu- rons in governing the transmission and modulation of nociceptive information. Our work would lead to a better understanding about DH circuits and provide potential new thoughts for chronic pain treatment.